Analytic estimates of the viscous time-scale due to cloud-cloud collisions have been as high as thousands of Gyr .
Consequently , cloud collisions are widely ignored as a source of viscosity in galactic disks .
However , capturing the hydrodynamics of discs in simple analytic models is a challenge , both because of the wide dynamic range and importance of 2D and 3D effects .
To test the validity of analytic models we present estimates for the viscous time-scale that are measured from three dimensional SPH simulations of disc formation and evolution .
We have deliberately removed uncertainties associated with star-formation and feedback thereby enabling us to place lower bounds on the time-scale for this process .
We also contrast collapse simulations with results from simulations of initially stable discs and examine the impact of numerical parameters and assumptions on our work , to constrain possible systematics in our estimates .
We find that cloud-collision viscous time-scales are in the range of 0.6 – 16 Gyr , considerably shorter than previously estimated .
This large discrepency can be understood in terms of how the efficiency of collisions is included in the analytical estimates .
We find that the viscous time-scale only depends weakly on the number of clouds formed , and so while the viscous time-scale will increase with increasing resolution , this effect is too weak to alter our conclusions .